1. Field of the Invention
The present invention generally relates to a fixing device fixing an unfixed image onto a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
An image forming apparatus such as a copier, a facsimile machine, a printer, and a printing machine has been used to obtain a copy or recorded medium by fixing (i.e., heating and pressing) an unfixed image onto a recording medium, the unfixed image having been transferred and carried on the recording medium. In the fixing, the recording medium on which the unfixed image is carried is heated and pressed while being fed and sandwiched, so that a developer, especially toner, included in the unfixed image is melted and softened to penetrate into the recording medium. By doing this, the toner may be fixed into the recording medium.
To perform fixing, it may be necessary to heat a fixing member to a predetermined temperature. Namely, to perform the fixing, it may be necessary to perform a warm-up operation. Whether the warm-up operation is finished is determined based on various methods, and one example of the methods is described below.
For example, Japanese Patent No. 3777722 (hereinafter “Patent Document 1”) discloses a fixing device in which a non-contact-type first temperature sensor is provided at a sheet feeding region of the fixing roller and a contact-type second temperature sensor is provided at a non-sheet feeding region of the fixing roller. The second temperature sensor at the non-sheet feeding region is used to determine whether the warm-up operation is finished, and the first temperature sensor at the sheet feeing region is used to control the temperature of the fixing roller. As described above, in Patent Document 1, determining whether the warm-up operation is finished is based on the temperature of the fixing roller.